Concerning sensor arrangements which detect the input rotation speed of a vehicle automatic transmission, Tokkai Hei 09-32918 published by the Japan Patent Office in 1997 discloses a rotation speed sensor comprising members which extend in a radial direction at intervals of a fixed angle from a clutch drum, and a sensor body facing these members. The sensor body outputs a signal whenever a member passes a specific position on a path. The rotation speed of the clutch drum is detected from the interval of this signal.